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| United States Patent |
6,204,479
|
|
Sickels
|
March 20, 2001
|
Thermistor protection for a wire feed motor
Abstract
A system for arc welding includes a welding power supply and a wire feeder.
The wire feeder includes a dc wire feed motor and a dc wire feed power
supply in electrical communication with the wire feed motor. A PTC
thermistor is disposed electrically between the power supply and the wire
feed motor. Current flows from the power supply, through the PTC
thermistor, and then to the motor. Under normal current conditions the PTC
thermistor allows current to be provided to the wire feed motor from the
power supply, but under excessive current conditions the PTC thermistor
inhibits current from being provided to the wire feed motor from the power
supply. A diode is connected in anti-parallel with the motor. A pulse
width modulation switch control current delivered to the motor. A normally
open relay is closed when welding is to be performed to provide current to
the motor, and a normally closed relay, connected in parallel with and
shunting the motor, is opened when welding is to be performed. The
normally closed relay acts as a brake on the motor when it is closed. The
PTC thermistor is not shunted by a resistor and/or a varistor, and/or is
not in parallel with a relay.
| Inventors:
|
Sickels; Darrell L. (Troy, OH)
|
| Assignee:
|
Illinois Tool Works Inc. (Glenview, IL)
|
| Appl. No.:
|
374067 |
| Filed:
|
August 13, 1999 |
| Current U.S. Class: |
219/137.71 |
| Intern'l Class: |
B23K 009/133 |
| Field of Search: |
219/137.71,137.7
361/25,27
|
References Cited
U.S. Patent Documents
| Re31367 | Aug., 1983 | D'Entremont.
| |
| 4119830 | Oct., 1978 | Gilliland | 219/137.
|
| 4408244 | Oct., 1983 | Weible.
| |
| 4856078 | Aug., 1989 | Konopka.
| |
| 5006778 | Apr., 1991 | Bashark.
| |
| 5264766 | Nov., 1993 | Tracht et al.
| |
| 5617001 | Apr., 1997 | Nacewicz et al.
| |
| 5793171 | Aug., 1998 | Hayashi et al.
| |
| 5990447 | Nov., 1999 | Nowak et al.
| |
| 6066834 | May., 2000 | Rebold | 219/137.
|
Other References
Hobart.RTM. Welding Products Handler 120/150 And Piecemaker 14A Gun Apr.
1999.
|
Primary Examiner: Shaw; Clifford C.
Attorney, Agent or Firm: Corrigan; George R.
Claims
What is claimed is:
1. A wire feeder for arc welding comprising:
a wire feed motor;
a power supply, in electrical communication with the wire feed motor;
a PTC thermistor disposed electrically between the power supply and the
wire feed motor.
2. The apparatus of claim 1 wherein under normal current conditions the PTC
thermistor allows current to be provided to the wire feed motor from the
power supply, and further wherein under excessive current conditions the
PTC thermistor inhibits current from being provided to the wire feed motor
from the power supply.
3. The apparatus of claim 1 wherein the motor is a dc motor and the power
supply provides current in one direction, and wherein the PTC thermistor
is disposed such that current flows from the power supply, through the PTC
thermistor, and then to the motor.
4. The apparatus of claim 3 wherein the power supply includes a pulse width
modulation switch disposed to control current delivered to the motor.
5. The apparatus of claim 4 further including a diode connected in
anti-parallel with the motor.
6. The apparatus of claim 5 wherein the switch is disposed such that
current flows from the motor, through switch, and then to the power
supply.
7. The apparatus of claim 6 further including a normally open relay that is
closed when welding is to be performed, and is disposed such when the
normally open relay is closed current flows from the power supply, through
the normally open relay, and then to the motors.
8. The apparatus of claim 6 further including a normally closed relay that
is opened when welding is to be performed, and is disposed such that when
the normally closed relay is closed it is connected in parallel with and
shunts the motor, whereby it acts as a brake on the motor.
9. The apparatus of claim 1 further including a normally closed relay that
is opened when welding is to be performed, and is disposed such that when
the normally closed relay is closed it is connected in parallel with and
shunts the motor, whereby it acts as a brake on the motor.
10. The apparatus of claim 1 wherein the PTC thermistor is not shunted by a
resistor and is not shunted by a varistor.
11. The apparatus of claim 1 wherein the PTC thermistor is not in parallel
with a relay.
12. The apparatus of claim 1 wherein the PTC thermistor is not shunted by a
varistor.
13. A wire feeder for arc welding comprising:
motor means for feeding wire;
power means for providing power to the motor means, wherein the power means
is in electrical communication with the wire feed motor;
a PTC thermistor means for protecting the motor means, wherein the PTC
thermistor means is disposed electrically between the power means and the
motor means.
14. The apparatus of claim 13 wherein the PTC thermistor means is not
shunted by a resistor and is not shunted by a varistor.
15. The apparatus of claim 14 further including a diode connected in
anti-parallel with the motor means.
16. The apparatus of claim 13 further including a normally closed relay
that is opened when welding is to be performed, and is disposed such that
when the normally closed relay is closed it is connected in parallel with
and shunts the motor means, whereby it acts as a brake on the motor means.
17. A system for arc welding comprising:
a welding power supply, disposed to provide power to an arc
a wire feed motor, disposed to provide wire to an arc;
a wire feed power supply, in electrical communication with the wire feed
motor;
a PTC thermistor disposed electrically between the wire feed power supply
and the wire feed motor.
18. The apparatus of claim 17 wherein under normal current conditions the
PTC thermistor allows current to be provided to the wire feed motor from
the wire feed power supply, and further wherein under excessive current
conditions the PTC thermistor inhibits current from being provided to the
wire feed motor from the wire feed power supply.
19. The apparatus of claim 17 wherein the PTC thermistor is not shunted by
a resistor and is not shunted by a varistor.
20. The apparatus of claim 17 wherein the PTC thermistor is not shunted by
a resistor.
21. The apparatus of claim 17 wherein the PTC thermistor is not shunted by
a varistor.
22. The apparatus of claim 17 wherein the motor is a dc motor and the wire
feed power supply provides current in one direction, and wherein the PTC
thermistor is disposed such that current flows from the wire feed power
supply, through the PTC thermistor, and then to the motor.
23. The apparatus of claim 17 wherein the wire feed power supply includes a
pulse width modulation switch disposed to control current delivered to the
motor.
24. The apparatus of claim 23 further including a diode connected in
anti-parallel with the motor.
25. The apparatus of claim 24 wherein the switch is disposed such that
current flows from the motor, through switch, and then to the wire feed
power supply.
26. The apparatus of claim 25 further including a normally open relay that
is closed when welding is to be performed, and is disposed such when the
normally open relay is closed current flows from the wire feed power
supply, through the normally open relay, and then to the motor.
27. The apparatus of claim 26 further including a normally closed relay
that is opened when welding is to be performed, and is disposed such that
when the normally closed relay is closed it is connected in parallel with
and shunts the motor, whereby it acts as a brake on the motor.
28. The apparatus of claim 17 wherein the wire feed power supply is the
welding power supply.
29. A system for arc welding comprising:
welding power means for providing power to an arc;
motor means for feeding wire to the arc;
wire feed power means for providing power to the motor means, wherein the
wire feed power means is in electrical communication with the wire feed
motor;
a PTC thermistor means for protecting the motor means, wherein the PTC
thermistor means is disposed electrically between the wire feed power
means and the motor means.
30. The apparatus of claim 29 wherein the PTC thermistor means is not
shunted by a resistor and is not shunted by a varistor.
31. The apparatus of claim 30 further including a diode connected in
anti-parallel with the motor means.
32. The apparatus of claim 29 including a normally closed relay that is
opened when welding is to be performed, and is disposed such that when the
normally closed relay is closed it is connected in parallel with and
shunts the motor means.
Description
FIELD OF THE INVENTION
The application relates generally to wire feeders used in arc welding, and,
more particularly, to protecting a wire feed motor.
BACKGROUND OF THE INVENTION
Many welding applications such as MIG (metal inert gas) or GMAW (gas metal
arc welding) utilize a wire feeder to provide filler metal to the weld.
Generally, the wire feeder will provide wire at a nominally constant
speed. A typical prior art wire feeder includes a motor that pulls wire
from a reel and feeds the wire at a wire feed speed to the weld arc. The
motor is controlled by a wire feed controller that may be a stand alone
controller or may be part of a controller that controls other aspects of
the welding process. The wire feed controller controls the speed of the
wire feeder and typically includes a potentiometer (or digital up/down
input buttons) on a front panel of the controller which the user uses to
set wire feed speed.
A trigger on the gun (torch) is pulled when the user wants to weld. A
trigger circuit causes power to be provided to the wire feed motor, and
wire is fed to the arc, along with welding power. When the user releases
the trigger, power is removed from the wire feed motor and the arc. Under
normal operating conditions the wire feeder provides the wire to the arc
and the current draw of the motor is within an acceptable range.
However, occasionally a feed problem such as the wire inadvertently being
welded to the gun tip, or becoming tangled, will cause the wire feed motor
to stall. The stalled motor will draw excessive current, and cause
overheating of the motor windings. This can damage the motor, or cause
other problems.
One known way to prevent motor damage from excessive current draw due to a
stall is to provide a fuse or fusible link electrically between the motor
and power source. When excessive current is drawn, the fuse opens the
motor power line. However, the fuse or fusible link needs to be replaced
prior to restarting the wire feeder, causing inconvenience and down-time.
One known protection device is a thermistor, which has been used in
non-welding applications. However, many non-welding thermistor
applications involve using the thermistor to control current through a
relay coil, and opening the coil in response to undesired high current.
This sort of scheme requires an additional relay, and may result in
excessive wear and tear to the relay.
Other non-welding thermistor applications involve using the thermistor as
both a protective element and a control element, wherein the thermistor is
used to inhibit current under extreme conditions, and controls the
magnitude of power provided under normal conditions. Such a scheme is of
little use for an application such as a wire feed motor having the power
controlled elsewhere.
One thermistor application involves using a thermistor for a start-up
circuit protection. The thermistor is shunted with a relay, and the relay
is closed after the start-up circuit has precharged components, and the
power source is connected to the proper input power, and the thermistor's
protective function ends. If the power source is connected to improper
input power the thermistor blocks the pre-charge, and the relay is not
closed. However, in an application such as a wire feed motor the excessive
current may occur at times other than start-up.
Accordingly, it is desirable to have a protective circuit for a welding
wire feed motor that is relatively inexpensive, unlikely to wear, useful
beyond start-up, and is not used to otherwise control power. Preferably,
such a circuit should not require user intervention to restart the motor
after the protective function is performed.
SUMMARY OF THE PRESENT INVENTION
According to a first aspect of the invention a wire feeder for arc welding
includes a wire feed motor and a wire feed power supply in electrical
communication with the wire feed motor. A PTC thermistor is disposed
electrically between the power supply and the wire feed motor.
Under normal current conditions the PTC thermistor allows current to be
provided to the wire feed motor from the power supply, but under excessive
current conditions the PTC thermistor inhibits current from being provided
to the wire feed motor from the power supply in one embodiment.
The motor is a dc motor and the power supply provides current in one
direction, and current flows from the power supply, through the PTC
thermistor, and then to the motor in another embodiment.
A pulse width modulation switch controls current delivered to the motor,
and/or a diode is connected in anti-parallel with the motor in various
alternatives. Current flows from the motor, through switch, and then to
the power supply in another embodiment.
A normally open relay is closed when welding is to be performed to provide
current to the motor, and/or a normally closed relay, connected in
parallel with and shunting the motor, is opened when welding is to be
performed, in various embodiments. The normally closed relay acts as a
brake on the motor when it is closed.
The PTC thermistor is not shunted by a resistor and/or a varistor, and/or
is not in parallel with a relay in alternative embodiments.
According to another alternative the wire feeder is part of a system for
arc welding that also includes a welding power supply.
Other principal features and advantages of the invention will become
apparent to those skilled in the art upon review of the following
drawings, the detailed description and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a welding power supply constructed in
accordance with the preferred embodiment; and
FIG. 2 is a schematic of a circuit implementing the present invention.
Before explaining at least one embodiment of the invention in detail it is
to be understood that the invention is not limited in its application to
the details of construction and the arrangement of the components set
forth in the following description or illustrated in the drawings. The
invention is capable of other embodiments or of being practiced or carried
out in various ways. Also, it is to be understood that the phraseology and
terminology employed herein is for the purpose of description and should
not be regarded as limiting. Like reference numerals are used to indicate
like components.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Generally, the present invention provides for protecting a wire feed motor
from excessive current with an in-line thermistor. The thermistor is
preferably not shunted, nor used for other control purposes. The invention
will be illustrated with reference to a particular protection circuit,
control circuit, power supply and wire feeder. It should be understood at
the outset that the invention may be implemented using other circuits,
power supplies, and wire feeders.
Referring now to FIG. 1 a block diagram of a welding system that implements
the present invention is shown. A MIG welding system 100 includes a wire
feeder 105 which is controlled by a controller 103. A MIG power supply 101
is also controlled by controller 103. In operation power supply 101
provides power to wire feeder 105. Wire feeder 105 feeds wire to an arc
107, at a rate determined by controller 103.
In accordance with the preferred embodiment welding system 100 may be of
the type sold commercially, such as a Hobart Handler.RTM. 120/150. Power
supply 105 receives input power via a 115/230 VAC receptacle.
Power supply 101, controller 103 and wire feeder 105 are shown as discrete
blocks in FIG. 1. However, in practice, controller 103 may be part of
power supply 101 or wire feeder 105. Additionally, all three blocks may be
contained within a single housing, and may be sold as a unit or
separately. In other alternatives controller 103 is distributed such that
part of it is in power supply 101, and part of it is in wire feeder 105.
Power supply 101 includes a power transformer such as that shown in FIG. 2.
(Other circuitry of power supply 101 is not shown). As seen in FIG. 2 the
primary side of the transformer has four taps and multiple windings
201-204, and the secondary has two windings 206-207. A rough control of
the output welding voltage is obtained by selecting one of the various
taps on the primary side of winding (which is connected to the 115/230 VAC
input). Primary windings 201-204 have 113 turns, 11 turns, 12 turns, and
13 turns, respectively. Secondary windings 206 and 207 each have 20 turns.
Thus, depending upon the tap selected, the peak secondary voltage will be
between 28.78 volts and 21.83 volts.
The secondary of the transformer is connected to a weld output power
circuit which includes a center-tapped full-wave rectifier circuit 210.
Two half-wave rectifier circuits are combined to use both half-cycles of
the secondary output voltage. A large electrolytic capacitor 212 (53,000
.mu.F) filters the full-wave rectified signal into a smooth DC signal. A
resistor 220 (50 ohms) is provided on the pc board to discharge capacitor
21. An output choke or inductor 214 (345 .mu.H) is provided to smooth
current flow to a pair of weld output studs 216 and 218. The core size,
and component values may be calculated in a conventional manner.
The circuit and topology of FIG. 2 is used in the preferred embodiment, but
any circuit, topology, and power supply may be used as well. Examples of
other arrangements with which this invention may be implemented include
(but are not limited to) convertors or invertors, phase control, control
by switching (not tap selection), AC output, CV output, etc.
In accordance with the present invention, a protective PTC (positive
temperature coefficient) thermistor 221 is provided to avoid damaging the
motor when a feed motor stall causes excessive current (10%, 20%, or more
excess current over the expected, typical, or rated current, e.g.).
The rectified output of rectifier 210 is also provided to a wire feed motor
225 through PTC thermistor 221. PTC thermistor 221 provides over-current
protection to the motor circuit. The holding current of PTC thermistor 221
is rated at 1.1A. The normal operating current of the motor while it's
feeding wire is 0.9A. If the motor is stalled due to a feed problem, it
will draw excessive current and cause PTC thermistor 221 to switch to a
high impedance state, effectively opening the motor circuit. PTC
thermistor 221 will remain in its high-impedance state until power is
removed from the circuit and the PTC is allowed to cool.
PTC thermistor 221 is in electrical communication with wire feed motor 225
because current can flow from PTC thermistor 221 to wire feed motor
(either directly or though other components) in series, parallel or other
combination with PTC thermistor 221 and wire feed motor 225). PTC
thermistor 221 is electrically between the power supply and wire feed
motor 225 because current that flows from the power supply to PTC
thermistor 221 flows to wire feed motor 225.
Also, PTC thermistor 221 does not have any control function: it only allows
current to pass when in its low impedance state, or inhibits (i.e.,
reduces to an acceptable level) current when its resistance rises.
PTC thermistor 221 is not shunted to ground by a resistor or varistor, as
many prior art thermistor protection circuits require (i.e., current does
not flow from the power source through PTC thermistor 221 and then to
ground through a resistor or varistor). Nor is PTC thermistor 221 shunted
by a resistor, varistor, or relay as many prior art thermistor protection
circuits require (i.e., not in parallel with). As used herein, shunted
means in parallel with or shunted to ground.
Motor 225 is a dc motor and the power supply is a dc power source, thus
current flows from the high output of the power supply (node 216) through
PTC thermistor 221, and through a normally open relay 223A, and then to
the motor (to the motor windings).
Normally open relay 223A is closed when the gun (torch) switch is closed,
and power is thus provided to the wire feed motor. Normally open relay
223A opens when the gun (torch) switch is released and power is thus
removed from the wire feed motor. A normally closed relay 223B is provided
to short the wire feed motor and provide a dynamic brake to the motor when
the contactor gun (torch) switch is released (magnetically braking the
motor). Normally closed relay 223B is opened when the gun (torch) switch
is closed, and the brake is thus removed. Normally closed relay 223B is
closed when the gun (torch) switch is opened, and the brake is thus
applied.
Feed motor 225 is connected to ground (which is also the low output of the
power supply) through a transistor 227 (part of controller 103), which
controls the turning on and off of feed motor 225. As will be explained
below, transistor 227 is a pulse width modulation switch, and controls the
speed of the motor by controlling current delivered to the motor. When
transistor 227 is open no current flows to the motor, and when transistor
227 is closed, current flows to the motor (windings)
A flyback diode 226 is in anti-parallel with motor 225 (i.e., antiparallel
to the direction of current flow from the dc power supply through the dc
motor windings). Diode 226 is provided across the motor winding to provide
a path for the energy to dissipate while transistor 227 is not on. An RC
network comprised of resistor 228 (3.92 K ohms), resistor 230 (1.00K ohms)
and capacitor 229 (0.001 .mu.F) protects transistor 227 from noise
generated by motor 225.
Some of the circuit described above, and the portion of the circuit
described below is part of the preferred embodiment, but not necessarily
needed.
Controller 103 includes a wire feed speed (WFS) control circuit which is
generally a pulse width modulated control. Greater pulse widths deliver
more power to motor 225, and result in a faster wire feed speed. The pulse
width modulation is implemented using a low-cost, industry-standard LM555
timer 234. Motor 225 runs fastest with the output of timer 234 at its
maximum pulse width. The output pulse of timer 234 (pin 3) is applied
through a diode 231 to the RC network comprised of resistors 230 and 228
and capacitor 229. In the preferred embodiment the current provided to the
motor is a speed control input because the speed of the motor is
responsive to the average current magnitude. The speed control input may
be a digital or analog control signal in embodiments where the motor
includes a controller.
Generally, timer 234 is configured in a conventional manner and its pulse
width is adjusted by a user selectable input such as a nonlinear
potentiometer 251 (0-50K ohms), or some other speed control input. The
potentiometer may be replaced with digital components such as an up/down
button and a microprocessor, or a potentiometer and a look-up table in
other embodiments). Potentiometer 251 is nonlinear in a manner so as to
compensate for other nonlinearities in the timing circuit, as will be
described in detail below. Potentiometer 251 is part of an input circuit
because it provides an input (a user input in the preferred embodiment) to
the controller. The input circuit may include other components, such as
filters, amplifiers, a/d convertors, etc.
The pulse width/timing may be understood beginning with node 248. The
signal at node 248 is the full-wave rectified line signal (60 Hz in the
United States), thus this signal goes to 0V every 8.3 mS. The signal at
node 248 is applied to the base of a transistor 246 through resistors 262
(10K ohms) and 260 (10K ohms). Each time the signal at node 248 drops
below 0.7V, a transistor 246 is switched off. Transistor 246 is connected
to the base of a transistor 257 through a resistor 255 (10K ohms) (and to
a regulated 15V supply through a resistor 253 (10K ohms)). Thus, when
transistor 246 is turned off, transistor 257 turns off. This removes the
voltage across a resistor 244 (10K ohms), which is connected to the
trigger input (pin 2) of timer 234. When the signal at node 248 rises
above 0.7V and switches transistor 246 on, transistor 257 is switched on,
which applies 15V across resistor 244. This creates a trigger pulse for
timer 234 at pin 2 which is synchronized to the AC line at 120 Hz.
Each time timer 234 is triggered by a low signal at pin 2, the output (pin
3) goes high for a time determined by an RC combination of potentiometer
251, resistor 242, (10.0K ohms) and capacitor 240 (0.1 .mu.F). Capacitor
240 will charge from the +15V supply through potentiometer 251 and
resistor 242 ohms) when a trigger occurs, until the voltage at pins 6 and
7 of timer 234 reaches the threshold voltage (2/3 V.sub.cc) or 10V, and
then capacitor 240 discharges (through timer 234). The output of timer 234
(pin 3) will switch to a high state while capacitor 240 is charging and
will remain high until capacitor 240 discharges.
Any signal which is created as part of the timing circuit may be considered
an intermediate control signal. For example, the voltage across resistor
242, or the voltage at pins 6 & 7 of timer 234 may be considered
intermediate control signals.
With potentiometer 251 adjusted to its minimum (shorted-out), the charging
time, and thus the pulse width and motor speed, is at a minimum. As the
WFS control (potentiometer 251) is rotated to maximum, the charging time
of capacitor 240 is increased as resistance is added into the circuit, and
the pulse width (and hence the motor speed) increases. With potentiometer
251 adjusted to its maximum, the charging time, and thus the pulse width
and motor speed, is at a maximum.
Alternative embodiments include using a controller having all analog or
predominantly digital circuitry. The timer circuit should be ideally
linear, but the components used to implement the timing, and the non-ideal
nature of real circuits, introduce nonlinearities into the PWM control.
Thus, while the PWM circuit may be inexpensive it is nonlinear, or is a
nonlinear stage.
Given the applications and processes for which the preferred MIG welding
system is likely to be used, a linear response of wire feed speed relative
to potentiometer setting is desired. Thus, in accordance with the
preferred embodiment the nonlinear nature of the timing circuit is
corrected by a nonlinear pot. Specifically, potentiometer 251 is created
to be nonlinear in such a way as to compensate for the nonlinearity of the
remainder of the timer circuit, i.e. the response of the timer circuit
(excluding potentiometer 251 such as to an intermediate control signal at
pins 6 & 7 of timer 234.
The desired resistance for various angular positions of potentiometer 251
was determined first by calculation, and then refined empirically, and was
selected to provide an overall substantially linear response. However, the
desired resistance could be determined in other ways, and could be chosen
to provide other than a linear over response.
A nonlinear potentiometer may be purchased commercially, or may be
specially made. The preferred potentiometer is similar to a linear
potentiometer in that it has a partial annular (arcuate) shape and the
wiper is turned by turning a knob on the front panel. The resistance is
proportional to the width of the annulus, and it's nonlinearity is created
by a changing width. The preferred potentiometer is created by laser
trimming (i.e. cutting to a desired width) to create the desired width
annulus. The changing width of the annulus may be smooth to produce a
gradually changing (varying) response slope, or it may have a step change
in width to create abrupt or step changes in response slope.
Thus, the input circuit of a controller can be made intentionally nonlinear
to correct for nonlinearities elsewhere in the controller. This is
preferably done with a nonlinear potentiometer that may be easily made and
relatively inexpensive. Because the nonlinear potentiometer may correct
for nonlinearities in the remainder of the control circuit, the remainder
of the control circuit may be inexpensively made, using relatively few
components.
Numerous modifications may be made to the present invention which still
fall within the intended scope hereof, such as implementing the invention
on a system used for other welding processes, providing an overall
response that is intentionally nonlinear, precisely linear, providing a
controller that is part of the wire feed motor (including an intermediate
control signal).
Thus, it should be apparent that there has been provided in accordance with
the present invention a method and apparatus for protecting a wire feed
motor with a PTC thermistor that fully satisfies the objectives and
advantages set forth above. Although the invention has been described in
conjunction with specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to those
skilled in the art. Accordingly, it is intended to embrace all such
alternatives, modifications and variations that fall within the spirit and
broad scope of the appended claims.
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